In many plating processes, a diffusion layer forms in the liquid electrolyte at the surface of the wafer. The diffusion layer reduces the mass transfer rate of electrolyte components and reactants to the surface of the wafer, which degrades the quality and efficiency of the plating process. One technique for increasing the mass transfer rate is to increase the relative velocity between the liquid electrolyte and the surface of the workpiece. In the past, some processing apparatus have used a paddle which oscillates horizontally or vertically in the electrolyte. The paddle has spaced apart ribs or blades. As the paddle moves, a liquid vortex is formed in the spaces between adjacent ribs. The liquid vortex creates a high speed agitated flow at or against the lower (down-facing) surface of the workpiece, increasing the mass transfer rate.
These types of paddle plating apparatus also often have an electric field shield provided to shield the edges of the wafer from the full electric field in the electrolyte, to achieve more uniform plating at the edges of the wafer. The shield is usually an annular ring of di-electric material.
Both the paddle and the shield are most effective when positioned very close to the wafer, for example, within 5 mm. If the shield is positioned below the paddle, the shield is less effective. If the shield is positioned above the paddle, then the paddle is less effective, as the gap between the paddle and the wafer is larger. Accordingly, engineering challenges remain in designing electro-plating apparatus.